fix Variable.advance() for multi-variable problems
fix Variable.advance() for multi-variable problems

- replace deque .data attribute by list

sfepy/sfepy,vlukes/sfepy,vlukes/sfepy,sfepy/sfepy,BubuLK/sfepy,BubuLK/sfepy,vlukes/sfepy,rc/sfepy,sfepy/sfepy,rc/sfepy,rc/sfepy,BubuLK/sfepy